In recent years, resins having extensive optical characteristics as compared with conventional optical resins have been demanded in various optical applications. In particular, high-refractive index materials are being energetically studied in view of the extent of their application ranges including various resin lenses for optical data storage for next-generation DVD, camera for mobile phone, etc., liquids for immersion microscope lens, optical recording materials and so forth. Above all, in optical recording materials, especially holographic recording materials, the development of a material capable of attaining high refraction and low shrinkage is of an important issue.
The holography is a terminology meaning the whole of techniques in which a pattern made by an interference fringe formed from two lights (object light or information light and reference light) is recorded on a photosensitive material, and when light is applied thereto at the same angle as in the reference light, a recorded image or information of the object can be read. In view of the fact that by treating light as a plane, recording and reading can be achieved in a wide portion within a short period of time, in recent years, the holography is a technique which is very expected as a high-capacity, high-speed recording material.
Though the holography is classified into several types from the standpoint of a recording mode of interference fringe, it is considered that phase holography (volume holography) is high in performance in view of high diffraction efficiency and wavelength selectivity.
As a material capable of achieving such a technique, an organic material using a monomer polymerizable with light, which is named a so-called photopolymer, is exemplified.
The photopolymer as referred to herein expresses a material composed of at least a matrix resin, a polymerizable reactive compound and a photoinitiator.
At the time of recording a hologram, when a recording layer is present in an area where two lights intersect to become strong, the photoinitiator causes a chemical reaction to convert into an active material, and this acts on the polymerizable reactive compound, whereby the reactive compound is polymerized. On that occasion, when there is a difference in a refractive index among polymers formed from the matrix resin and the polymerizable reactive compound, an interference fringe becomes the difference in a refractive index and is immobilized in the recording layer. Also, on the occasion when the polymerizable reactive compound is polymerized, diffusion of the reactive compound occurs from the surroundings, and concentration distribution of the reactive compound or polymer of the reactive compound is generated in the inside of the photopolymer. Though when only reference light is irradiated, a record on the basis of the object light or information light is reproduced from the interference fringe, in the case of using the reference light having the same wavelength, by irradiating it on the recording layer at the time of reproduction, an unreacted polymerizable reactive compound should be similarly polymerized at the time of recording. However, as described previously, in view of the fact that concentration distribution of the reactive compound or polymer of the reactive compound is generated in the inside of the photopolymer, in an area where the two light intersect to become weak, namely an unrecorded area, since the concentration of the reactive compound relatively decreases, even when the reactive compound is polymerized in this area, the difference in a refractive index from the recorded area still remains. Accordingly, even when the same wavelength is irradiated at the time of reproduction, the record does not vanish (Non-Patent Document 1).
As a representative polymerizable monomer as such a reactive compound for photopolymer, N-vinylcarbazole and tribromophenyl acrylate are investigated (for example, Patent Documents 1 and 2). These compounds are sufficiently large in a refractive index of a polymer formed upon being polymerized as compared with a matrix resin, high in compatibility with the matrix resin in a stage before the polymerization and also high in compatibility with the matrix resin even after the polymerization, since unnecessary scattering is small; and therefore, even when repeatedly recorded, the diffraction efficiency is high, so that these compounds are suitable for holographic recording.
Also, it is reported in Patent Documents 3 to 5 that by using a polymerizable sulfur atom-containing compound or the like, a holographic recording material and a recording medium each having high diffraction efficiency and sensitivity are obtained. However, in general, there is a trade-off relation between diffraction efficiency and rate of shrinkage, and when a concentration of a polymerizable compound is increased for the purpose of enhancing the diffraction efficiency, polymerization shrinkage following recording becomes large. Even polymerizable compounds of the working examples exemplified in these patent documents are insufficient from the standpoints of diffraction efficiency, light transmittance and rate of shrinkage, and creation of further polymerizable compounds is being demanded.